1. Field of the Invention
This invention relates to articles that form flow passages or otherwise contain hydrocarbon fluids at high temperatures. More particularly, this invention is directed to a surface coating for such an article and a method for forming the coating, wherein the coating inhibits the deposition and adhesion of hydrocarbon fluid thermal decomposition products, such as gum, coke, sulfur compounds and other impurities, on the article.
2. Description of the Prior Art
As is known in the art, hydrocarbon fluids are prone to the formation of thermal decomposition products at elevated temperatures. In the form of deposits, such decomposition products can foul heat exchangers, plug fuel injectors and lubrication distribution jets, jam control valves and cause problems with many other types of operating and control devices through which the hydrocarbon fluid flows.
The formation of hydrocarbon decomposition products is generally referred to as thermal instability or, in the case of hydrocarbon fuels, fuel instability. The mechanisms for formation of deposits from thermal instability have been studied and documented. In the case of fuels, it is generally accepted that there are two distinct mechanisms occurring at two temperature levels. In the first mechanism, referred to as the coking process, as temperature increases from room temperature, there is generally a consistent increase in the rate of formation of coke deposits up to about 675.degree. C. (about 1250.degree. F.), at which high levels of hydrocarbon pyrolysis lead to coke formation and eventually limit the usefulness of the fuel. A second lower temperature mechanism generally peaks at about 370.degree. C. (about 700.degree. F.) and involves the formation of gum deposits. This second mechanism generally involves oxidation reactions, leading to polymerization that includes the formation of gums. Coke and gum formation and deposition can occur simultaneously in the mid-temperature region.
Coke formation in hydrocarbons is discussed in U.S. Pat. No. 2,698,512, and heat stability of jet fuel and the consequences of thermal degradation of the fuel are discussed in U.S. Pat. No. 2,959,915, both patents being incorporated herein by reference. These patents suggest specific formulations which place limitations on the fuel chemistry and impurities associated with hydrocarbon fuels, so that the fuels will be usable at high temperatures without the typical formation of gums and coke. Gum and coke formation are also discussed in U.S. Pat. No. 3,173,247 to Smith et al., incorporated herein by reference. Smith et al. note that heat must be transferred from the engine to some part of the aircraft and/or its load when an aircraft is operated at very high flight speeds. Smith et al. further note that, though the fuel on the aircraft could serve to receive this heat, to do so is unfeasible because jet fuels are not stable at the high temperatures that are developed at multi-Mach speeds, but instead decompose to produce intolerable amounts of insoluble gum or other deposits, such as coke. As with the previously referenced patents, the solution proposed by Smith et al. is to place limitations on fuel chemistry and impurities associated with the fuel.
Even with the most elaborate special treatment of the fuel, coke formation cannot be entirely eliminated even when a pure hydrocarbon is used because coke formation will always occur given a sufficient temperature and duration. On the other hand, the chemistry of the hydrocarbon fluid mixture and the chemistry of the vessel used to contain the fluid can have a major influence on deposit mechanisms and deposit rates at the temperatures where it is most desirable to use the fluid. In the lower temperature region where gum formation occurs, oxygen from air dissolved in the liquid is the major adverse ingredient. Boiling amplifies this adversity because of the oxygen concentration effect of gas bubbles adjacent to hot walls. If oxygen or air is absent, gum formation is less likely to occur.
In much of the prior art, the problems associated with gum and coke thermal deposits have predominately dealt with bulk fluid chemistry and reactions that may take place within the fluid. These investigations have involved a wide range of hydrocarbon compositions and the presence of numerous impurities such as sulfur compounds, nitrogen compounds, oxygen and trace metals. It has been observed that deposits attached to the walls of the fluid vessel often contain very large quantities of sulfur compounds or intermediates thereof, in addition to gums and cokes. Little attention has, however, been given in the prior art to the role of the chemistry and reactions which directly take place between the vessel walls and the fluid. Even though wall reactions are not well understood, it has been proposed that fluid-wall deposit thickness reactions might be reduced if the wall were coated with some form of relatively inert material. For example, U.S. Pat. No. 4,297,150 to Foster et al. teaches a protective metal oxide film for metal surfaces that are susceptible to coking, corrosion and catalytic activity. As is conventional with metal oxide films, Foster et al. require that, prior to deposition of the oxide film by chemical vapor deposition (CVD) using a carrier gas, the metal surfaces must be oxidized in order to achieve an adherent and uniform film. Though Foster et al. do not disclose any other surface preparations, one skilled in the art would appreciate that efforts to reduce the surface roughness of the metal surface would be antithetical to the teachings of Foster et al., since the surface of the oxidized metal surface is intended to promote adhesion of the oxide film. Conventional surface finishes for promoting the adhesion of metal oxide films are generally on the order of at least about 4.0 micrometers (about 160 microinches) R.sub.a, as evidenced by U.S. Pat. No. 4,942,732 to Priceman.
Thermal instability and fuel instability are becoming more significant with developing technology, and become even more significant as operating temperatures increase through advances in materials technology and as the chemical quality of hydrocarbons for fuels, oils, lubricants, petrochemical processes (plastics and synthetics) and the like, decreases. Furthermore, hydrocarbon fluids, fuels and oils derived from nonpetroleum sources, such as shale and coal, will have significantly more problems with thermal instability because of their high content of olefins, sulfur and other compounds.
Accordingly, it would be advantageous to provide fluid containment articles and processes for preventing the formation of adverse decomposition products and foulants in such applications where thermal instability, including fuel instability, is a problem as a result of exposure to such fluids to high temperatures. In particular, it would be desirable if such advancements were achieved with fuel containment articles for holding or otherwise containing the flow of a hot hydrocarbon fluid, including heat exchangers that use a hydrocarbon fuel as the coolant and fuel nozzles that deliver a hot hydrocarbon fuel to a gas turbine engine combustor where the fuel is burned, so as to prevent or significantly reduce the tendency for insoluble gums, coke, sulfur compounds or mixtures thereof to deposit and adhere to surfaces of the article. Such capabilities would preferably be achieved without altering the hydrocarbon chemistry, maintaining strict control of impurities and/or providing additives and special processing such as passivation treatments, because such techniques constrain the use of the fluid, increase cost and promote uncertainty as to the quality level of the fuel or treatment at a particular time.